Gallium nitride and aluminum gallium nitride are wide bandgap semiconductors used in the production of such electrical and opto-electronic devices as blue light emitting diodes, lasers, ultraviolet photodetectors, and power transistors. There are currently no cost effective, lattice matched substrates on which these crystalline materials can be grown. Common exemplary substrates for the growth of these materials are sapphire, silicon, gallium arsenide, and silicon carbide. Each of these materials has significant lattice size differences with respect to the gallium nitride (GaN) or aluminum gallium nitride (AlGaN) crystal structure. For example, the lattice size differences for gallium nitride on sapphire is 16%, gallium nitride on silicon carbide is 3.1%, and gallium nitride on silicon is 17%.
The lattice mismatch between the substrate and the epitaxial overgrown layer is accommodated by a defect in the periodic crystal structure of the epitaxial layers. This defect is called a dislocation. Dislocation densities above 104 cm−2 degrade performance of both optical and electronic devices by carrier scattering, catalyzing impurity movement, roughening interfaces, and serving as a parasitic defect/recombination site. In order to preserve smooth interfaces and reduce dislocation densities, a variety of mitigation and density reduction approaches have been proposed in the past.
One of the more well-known approaches uses lateral epitaxial overgrowth. Essentially, the underlying substrate is patterned using a photomask and material is grown in windows opened to the substrate. As the crystal grows, the window tends to overgrow the masked area. In this overgrown area, the threading dislocation density is significantly lower than in the window area (up to 4 orders of magnitude lower). One example of a process such as this can be seen in U.S. Patent Application Publication No. 20010008791 (Gherke, et al.).
The disadvantage of this technique is that the substrate must be patterned and, in turn, area is sacrificed. As a result, there is a need for methods of fabrication and resulting devices that have reduced lattice mismatch between layers.